Transmitting and receiving device, terminal device, and transmitting and receiving system

A transmitting and receiving device includes a controller, a driver, a specific pattern generator, a transmitting signal detector, an amplifier, a differential amplifier, an average current detector, and a received signal detector. In a non-signal period, the controller causes a current signal to be input from the driver to a laser diode and causes an optical signal to be output from the laser diode. When an optical signal of a specific pattern output from the other-side laser diode reaches a photodiode over a period of length that depends on an average value of a current signal output from the other-side photodiode that receives the optical signal, the controller adjusts a magnitude of the current signal input from the driver to the laser diode based on the length of the period of the optical signal of the specific pattern.